The present invention relates to pumping systems for producing well fluids from petroleum producing formations penetrated by a well and, in particular, to a method and apparatus for preventing plugging of an oil well pump in a well which produces fluids containing suspended particles.
Oil wells which penetrate formations that produce heavy crude oils often produce fluids laden with sand. A recurring problem in the production of well fluids from such wells is the plugging, jamming or seizing of bottom hole pumps. Most wells exhibit a threshold rate at which formation fines become mobile in the formation and are produced because of drawdown and velocity of the fluids being produced. The threshold rate in unconsolidated sand formations is very low. The problem is acute when heavy oil is produced because it is usually found in shallow, unconsolidated sand formations and the sand is often too fine to screen out without plugging the screen or limiting throughput to an unacceptable degree. For example, heavy viscous crudes which are relatively close to the earth""s surface contain sand and are difficult to pump. Steam and diluents have often been used to lower the viscosity of such heavy crudes to improve flow and pumping efficiency. However, sand is still a major problem.
Most of the oil fields in Canada have been exploited for a long period of time and are relatively old, so they now produce over 90% water. In the United States, most of the on-shore fields are in an even more advanced state of decline. The wells that are currently being drilled in Canada and the United States enter a state of decline in a much shorter time than the wells which were drilled in the past. On average, a well drilled today has a service life of about five years from the first production to abandonment. Wells which produce high percentages of water must be produced at a high rate to yield enough oil to be economic. Thus, most wells are now being produced over the threshold rate discussed above and sand is produced with the water/crude mixture.
Another source of sand production in wells is fracture propant which is injected into a producing formation during a process well known in the industry as hydraulic fracturing, which is used to increase the productivity of wells.
People in the industry have made efforts to overcome this long-standing problem. Different types of pumps and special pumping systems have been suggested for improving the productivity of well fluids containing suspended particles. Progressing cavity pumps (PCP) are the most successful type of pump in common use in wells today for pumping sand laden crude oil because the PCP is an excellent sludge pump. However, when a PCP is stopped during production of sand-laden crude, sand suspended in the production tubing settles out of suspension and may plug the production tubing. It takes less than a metre of sand to form a plug which cannot be breached with the pressure available from the pump. Furthermore, the pump is usually driven backwards when it is stopped because of the draining of the well fluids in the production tubing string through the pump. If this happens, the sand plug may extend into the pump and seize it.
The problems described above are not unique to PCPs and are actually more severe with other types of pumps. For example, sand plugging problems occur to pumping systems using dual tubing strings which have been suggested to be used for pumping heavy viscous and sand laden oil. An improved parallel tubing system for pumping well fluids is described in U.S. Pat. No. 5,505,258, entitled PARALLEL TUBING SYSTEM FOR PUMPING WELL FLUIDS which issued to Muth on Apr. 9, 1996. Muth teaches a production tubing between the earth""s surface and a production zone in the well for receiving well production fluids from a pump located in a power tubing. A flow control valve is located in the lower end of the production tubing to permit the flow of the production fluids up to the earth""s surface and prevent the flow of production fluids down through the production tubing. The flow control valve is located in close proximity to the pump. The power tubing extends down the well in a parallel relationship with the production tubing to the production zone in the well. Production fluids pumped by the pump are down into a lower portion of the power tubing. The pump is located in the lower portion of the power tubing and driven by a pump rod string that extends down through the power tubing. A cross-over flow path is formed between the low portion of the power tubing and the production tubing below the flow control valve for flowing production fluids out of the power tubing and into the production tubing, the production fluids then being transferred to the earth""s surface through the production tubing.
Although the system described by Muth has many advantages and does prevent sand from plugging the pump if there is an interruption in the operation of the pump, it does not provide a solution for restarting production after sand has settled out above the valve. As pointed out by Muth, a pump is sanded up or stuck in the pipe because of sand settling out of the production fluids on top of the pump whenever the well is idle for short periods of time. When this happens, an unprotected pump has to be retrieved from the well if the production is to be resumed. In a conventional system, the tubing string and sucker rods usually are pulled xe2x80x9cwetxe2x80x9d, because the clogged tubing cannot be drained, which is not only an awkward operation, it can also cause objectionable oil spills on the surface. With the system described by Muth, however, the wet operation can be avoided regardless of how much sand the well is production because a sand plug in the production tubing can be cleaned out with coil tubing or bailing or other methods known in the art. Nevertheless, such cleanout takes time and is not without expense.
Consequently, it is desirable to have an apparatus that will prevent particles that settle out of well fluids from clogging a pump when the pump is idled for a short period of time. It is also desirable, if the apparatus permits, particles to be resuspended after they settle out of well fluids and plug the production tubing so that the resuspended particles can be pumped up out of the production tubing and production can be easily resumed after the production is idled.
An object of the invention is to provide an apparatus for preventing plugging of an oil well pump in a well which produces fluids containing suspended particles.
Another object of the invention is to provide a method of resuspending particles that settle out of well fluids in a production tubing of a well during an idle period of the pump, to permit production from the well to be restarted.
A further object is to provide a simple and economical solution for preventing clogging of an oil well pump in a well which produces fluids containing suspended particles, and resuspending the particles that settled out from well fluids in a production tubing during a period that the pump was in idle.
In accordance with one aspect of the invention a method of preventing plugging of an oil well pump in an well which produces fluids containing suspended particles comprises setting a valve in a production tubing above the pump, the valve permitting well fluids to flow up past the valve while inhibiting the well fluids from flowing back past the valve, the valve being positioned to trap a volume of the well fluids between a bottom of the valve and a top of the pump adequate to permit a pressure wave to be developed between the pump and the valve when the pump is restarted from an idle condition; and after a period during which the pump was idle and the particles settled from the well fluids above the valve, starting and stopping the pump causes the pressure wave in the well fluids to force the well fluids past the valve until the particles that have settled are resuspended to an extent adequate to permit production through the production tubing to resume.
An apparatus for application of the method preferably includes a valve insertable into and removable from a production tubing above the pump so that the trapped volume of well fluids between the bottom of the valve and the top of the pump is adequate to permit a pressure wave induced by a starting of the pump to develop. Preferably, the valve is positioned about 20-40 meters (60-125xe2x80x2) above the pump. To ensure uninhibited operation of the valve, all linkages between the pump and ground equipment are preferably routed outside of the production tubing.
The advantage of the invention lies in a simple, low cost solution to a long standing problem which previously required pulling of the production tubing from the well in a xe2x80x9cwetxe2x80x9d condition in order to remove particles so that production could be restarted, or in the case of the rodless systems, inserting a coiled tubing or a bailing tool to clear sand trapped above the standing valve.